Problems of implementation of pharmacogenomics achievements

Pharmacogenetics is quite often talked about today as a new direction. However, the term "pharmacogenetics" was proposed by Friedrich Vogel back in 1959. The subject of pharmacogenetics is the influence of genetic characteristics of the body on the effects of drugs. The decoding of the genome and the development of methods of molecular biology contributed to the emergence of a new term – pharmacogenomics, which is essentially synonymous with the previous one, but more reflects modern methodology.

Pharmacogenetic research has been developing very intensively over the past 50 years. Nevertheless, the achievements of pharmacogenetics and pharmacogenomics are being introduced into practice extremely slowly. Moreover, this problem is relevant all over the world.

At the initial stages, pharmacogenetic differences, primarily in pharmacokinetics, were considered as an interaction (the result of interaction?) a particular drug with one or more genes. This approach has led to the establishment of the polymorphic nature of many enzymes involved in drug metabolism, as well as transporters responsible for the penetration and excretion of drugs and their metabolites from cells. Thanks to this approach, enzymes have been identified, the interaction with which should be established at the preclinical stage of the study of new drugs.

New knowledge for practitionersFor 50 years, sufficient material has been accumulated that allows us to use the achievements of pharmacogenetics in practical medicine today.

This concerns, first of all, taking into account the phenotype of biotransformation to optimize pharmacotherapy.

Polymorphic variants of cytochromes providing the first phase of metabolism of more than 50% of drugs used in clinical practice have been studied in the most detail. The lists of drugs with side effects include 59% of drugs that are metabolized by phase I polymorphic enzymes, of which 86% are cytochromes. There are more than 200 variants of cytochrome alleles involved in drug metabolism. The largest amount was found for CYP2D6 – 46, for CYP2C9 – 12, for CYP2C19 – 16. Different oxidation phenotypes determine differences in pharmacokinetic parameters.

About 25% of all medications are CYP2D6 substrates, including beta-blockers, tricyclic antidepressants, neuroleptics, morphine derivatives and other drugs, many of which have a small therapeutic breadth. Using the methods of genotyping and phenotyping, the approximate percentage of variants of cytochrome CYP2D6 activity in the white population was determined: 50% – homozygotes for the "wild" allele – have normal enzyme activity, 40% – with one active allele – intermediate, 7% – "slow metabolizers" – homozygotes for the null allele, and 3% – "ultrafast metabolizers" – have several copies of the gene. Ethnic differences in the frequency of polymorphisms are very pronounced. For example, in Ethiopia, 29% of "ultrafast metabolizers", and in China – less than 1%. It has been proven that differences in enzyme activity can lead to significant variability in the efficacy and safety of substrate drugs when using the same dose in different patients.

For example, pronounced differences in the pharmacokinetic parameters of metoprolol and timolol beta-blockers in "fast" and "slow" metabolizers according to CP2D6 were determined. A decrease in the metabolic rate leads to the formation of high concentrations of drugs in the body and significantly increases the risk of side effects. The same group includes sotalol and metoprolol, the metabolism of which does not depend on CYP2D6. Thus, it is obvious that information about metabolism involving polymorphic enzymes must necessarily be contained in the information about the drug. This will allow the doctor, even in the absence of opportunities for pheno-(geno)typing of the metabolizing enzyme, taking into account possible pharmacogenetic features, to carry out the selection of drugs.

The next clinical example is the correction of the daily dose of nortriptyline depending on the phenotype of oxidation by CYP2D6 from 10 to 500 mg / day. In this study, the activity of the enzyme was determined by phenotyping by the metabolic ratio of debrisochine, which was used as a typing drug. When using a standard dose without taking into account pharmacogenetic differences, side effects may develop in "slow metabolizers", and in "ultra–fast" ones, the therapeutic effect may not be achieved.

The opposite situation may occur when an active metabolite is formed with the participation of a polymorphic enzyme. For example, CYP2D6 is involved in the formation of the active metabolite of codeine – morphine. In this case, with low enzyme activity, the analgesic effect of codeine is not achieved, and with high activity, the risk of addiction increases.

Differences in pharmacokinetics due to the genetic characteristics of the activity of the metabolizing enzyme can be significantly enhanced by drug interaction when several drugs are used that are substrates of the same metabolic enzymes.

One of the simplest and least expensive pharmacogenetic approaches that allow for correction of drug doses and optimize pharmacotherapy is the determination of enzyme activity using typing drugs. For most polymorphic enzymes, such preparations have been identified. The requirements for such drugs are relative safety, simple metabolism with the participation of a typed enzyme, simple determination of the metabolite in urine. This method has a number of limitations, since not only genetic factors can affect the metabolism of drugs, and it is quite difficult to take them all into account. The existence of phenocopies confirms the expediency of combining the methods of phenotyping and genotyping to rationalize pharmacotherapy. It should be noted that such methods have already been developed for most enzymes and are becoming increasingly available.

Cytochrome CYP2D6 polymorphism has been known for more than 20 years, however, drugs metabolized with the participation of this enzyme continue to be prescribed in a small range of standard doses, selected for most people in the white population. The same applies to almost all established polymorphisms of the enzymes of metabolism of the 1st and 2nd phases and transporters involved in the processes of absorption and excretion. In addition to cytochromes CYP2D6, CYP2C9, and CYP2C19, polymorphisms of the genes of thiopurine methyltransferase (TPMT) and N-acetyltransferase (NAT) metabolism enzymes metabolizing cytostatics and anti-tuberculosis drugs, respectively, are very significant. The necessary correction of doses of known drugs for "slow", "intermediate", "fast" and "ultrafast" (according to CP2D6) metabolizers ranges from 10 to 180% of the standard dose.

The main obstacle to the introduction of pharmacogenetic testing into clinical practice is the lack of understanding of the need to determine the activity of the enzyme to rationalize pharmacotherapy, and often simply lack of knowledge in the field of pharmacogenetics. Therefore, in our country, it is necessary to increase the number of studies proving such a need, including pharmacoeconomical and pharmacoepidemiological studies, taking into account the frequency of polymorphisms in the population. It is extremely relevant to develop and implement simple and accessible typing methods and sound recommendations for correcting dose regimens depending on the results obtained. In accordance with the results of pharmacogenetic studies, information about drugs should be updated. For example, in the USA, information about the widely used anticoagulant warfarin, the number of prescriptions of which in the USA is about 2 million per year, has been revised 20 times since 1954 (for example, interaction with cranberry juice was established for the penultimate time). And in 2005, the US licensing system (Food and drug administration, FDA) recommended an assessment of the activity of cytochrome C2C9, which provides metabolism, and the polymorphism of the vitamin K epoxide reductase VKORC1 gene, which changes the interaction with the target. It has been proven that typing on these signs before prescribing warfarin has a high predictor value, allows you to choose the right dose in the range from 0.5 to 8 mg / day. and increase the effectiveness and safety of therapy. At the same time, special programs have been developed to help the practitioner choose the correct dose of the drug based on the data obtained for a particular patient, including typing results, as well as taking into account age and body weight (http://www.warfarindosing.org /).

Research prospectsSpeaking about the modern possibilities of pharmacogenetics in assessing pharmacodynamic features, it should be noted that the evidence of the fact that the effect of a drug cannot be controlled by one or more single genes.

The response to the drug is a complex genetic problem affecting dozens of genes. These genes can encode a pharmacological target or signaling molecules located behind it. The contribution of each individual gene to individual differences may be minimal, they may depend on the interaction of genes with each other and with environmental factors. Thus, a more complex approach is required to cover the entire genome, it is necessary to use pharmacogenomics and bioinformatics methods.

Most diseases are polygenic and multifactorial and manifest themselves in various phenotypic variants. Therefore, the main approach to individualization of pharmacotherapy is to create drugs for certain groups of patients or even for a specific patient. The search for individual targets is impossible without the use of pharmacogenetic methodology at the stage of drug development. Selection of phenotypes for genetic analysis, analysis of a complex phenotype, confirmation of heritability, pharmacogenomic analysis, identification of genetic variants associated with the phenotype – all this allows us to identify candidate genes regulating known links of the pathophysiological process or involved in the molecular mechanism of action of drugs used to treat this disease. Evaluation of polymorphisms in these genes, comparison with the phenotypes of the disease and, most importantly, the establishment of the functional significance of the identified variants allow us to begin the development of new drugs that selectively affect the identified targets.

An important task of pharmacogenomics, along with the creation of means of personalized pharmacotherapy, is the development of DNA tests that allow with a high degree of sensitivity and accuracy to determine the effectiveness and toxicity of drugs before the patient starts taking the drug. At the same time, it is important to understand that such an analysis cannot be absolutely predictive, but will provide only probabilistic information. Therefore, conducting studies proving the medical and economic feasibility of introducing pharmacogenetic testing for certain drugs and groups of patients is extremely relevant.

Clinical studies in the perspective of genotypingEvaluation of safety and efficacy, testing of dose regimens occur at the stage of clinical trials.

It is at this stage that the largest number of drugs being created is eliminated. Often the cause is a large variability of the main and side effects and, as a consequence, the absence of statistically significant positive results in limited samples of patients. This not only leads to losses for drug developers, but also deprives certain groups of patients of potentially effective drugs for them. There are a significant number of publications in which it is reported that according to the results of multicenter clinical trials, drugs were recognized as ineffective, although an individual assessment could identify groups of patients who had an improvement in their condition.

Taking into account the data of pharmacogenomic studies obtained at the preclinical stage when planning each phase of clinical trials will increase their effectiveness and, possibly, reduce the cost and duration. The phase 1 study should include participants with genotypes identified as optimal for the effect of this drug. Analysis of phenotypes of reactions to the drug and data on the evaluation of polymorphisms of candidate genes providing pharmacokinetics and pharmacodynamics (in phase 2), preliminary typing of phase 3 participants will determine the characteristics of patients responding and not responding to treatment, as well as subpopulations more susceptible to the development of side effects. In multicenter phase 4 studies, a decision may be made on the feasibility of introducing diagnostic pharmacogenomic tests to predict the effectiveness of this drug. It is necessary to store DNA samples for meta-analysis of the causes of possible complications and create data banks.

In Russia, the task of introducing pharmacogenetics into practice, including the creation of pharmacogenetics laboratories in large medical institutions, is assigned to doctors – clinical pharmacologists by order of the Ministry of Health (No. 494 of 10/22/2003). However, the execution of this order, both in terms of the organization of laboratories and the training of specialists, can be considered unsatisfactory. It is obvious that knowledge of pharmacogenetic and other laws of the action and interaction of drugs is necessary for all practitioners prescribing well-known drugs and planning the clinical study of new drugs, as well as specialists in biomedical and pharmaceutical profile.

In 2002, the first Department of Pharmacogenetics in the country started working at the Faculty of Medicine and Biology of the Russian State Medical University. The department was established on the basis of the Pharmacogenetics Laboratory of the V.V.Zakusov State Research Institute of Pharmacology of the Russian Academy of Medical Sciences (Head – Academician of the Russian Academy of Medical Sciences S.B.Seredenin). The course of pharmacogenetics is taught at the Department of Clinical Pharmacology of the I.M.Sechenov Moscow Medical Academy (Academician of the Russian Academy of Medical Sciences V.G.Kukes), in some universities of the country. This is clearly not enough. Therefore, today the task of all specialists involved in pharmacogenetics is to promote this ideology, to unite doctors, scientists, health care organizers, pharmaceutical and biotechnology companies.

Evaluation of pharmacogenetic features of metabolic enzymes, transporters and pharmacological targets using phenotyping and genotyping methods is the basis for individualization of pharmacotherapy, since it allows determining the appropriateness of prescribing a particular drug, choosing the optimal dose and application scheme, substantiating the possibility of using a complex of drugs, as well as creating new drugs for personalized therapy.

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26.11.2008